Metal coatings, particularly of tin or its alloys are frequently deposited onto surfaces of copper- and nickel-based workpieces in order to prevent the copper or nickel-based surface from oxidizing or tarnishing and to enhance solderability.
Under conditions such as elevated temperatures in air or in other oxidizing atmospheres, for example tin or other metal-coated surfaces of electronic lead frames and electrical connectors have a tendency to form oxide films during periods of shipment and storage between manufacture and assembly into electronic devices. The oxide coats discolor the surface of the tin-coated surface and impart a yellowish color which many consumers consider unacceptable. Furthermore, the oxide may degrade the contact resistance of a coated electrical terminal. A tarnish-free surface has lower electrical contact resistance and better solderability than an oxide coated surface.
Tin-based layers are also used in lead finishes for integrated circuit (“IC”) fabrication. A thin layer of tin or tin alloy is applied as the final step for passive components such as capacitors and transistors.
There are many factors that determine good solderability, the three most important of which are extent of surface oxide formation (corrosion), amount of codeposited carbon, and extent of intermetallic compound formation. Surface oxide formation is a natural occurring process because it is thermodynamically favorable. The rate of formation of the surface oxide depends on the temperature and time. The higher the temperature and the longer the exposure time, the thicker the surface oxide formed. In electroplated tin or tin alloy coatings or deposits, surface oxide formation also depends on the surface morphology of the coating or deposit. When comparing pure tin to tin alloy coatings, for example, tin alloys generally form less or thinner surface oxides when all other conditions are equal.
Generally, it is the aim to produce a tarnish-free surface, which has lower electrical contact resistance and better solderability than an oxide coated surface.
Fuchs, et al., in U.S. Pat. No. 5,853,797 disclose a method and solution for providing corrosion protection of coated electrical contact surfaces which involve exposure of such surfaces to a solution containing phosphonates, lubricants and various volatile organic solvents. Evaporation of such solvents for disposal is fraught with environmental concerns such as handling, hazard to workers, and disposal of waste into streams.
Fan, et al., in U.S. Pat. Appl. No. 2005/0268991 A1 disclose a method for enhancing corrosion resistance of a tin-based surface on a workpiece involving contacting the tin-based surface with a composition comprising a phosphonic acid compound and water to form a phosphorus-based film over the tin-based coating thereby inhibiting corrosion of the tin-based surface. Phosphonic acid containing compositions having a concentration up to about 30 vol. % of an organic solvent, and water. However, the use of organic solvents is disadvantageous since they are volatile under process conditions and often hazardous.
Lau, et al., in U.S. Pat. Appl. No. 2006/0237097 A1 disclose a method for inhibiting corrosion of metals and metal alloys by treating them with compositions containing inorganic and organic phosphoric acids like alkylphosphoric acids that prevent oxide formation on the metals and metal alloys.
EP 0 009 247 B relates to the use of alkyl monophosphonic acid containing from 8 to 10 carbon atoms in admixture with surfactants as corrosion inhibitors against metals in acid solutions. The metals are selected from the group consisting of aluminum and aluminum alloys, chromium-nickel-steal, ordinary steal, brass and copper.
EP 1 221 497 A2 relates to a method for inhibiting the formation of stains, especially water stains, on the exterior surfaces of aluminum alloy products. The method entails contacting the exterior surfaces of these products, particularly sheet or plate products, extrusions and/or forgings made from 5000 to 6000 Series aluminum alloys, with an organophosphonic or organophosphinic acid-derived material. Preferably, liquid forms of this material are added to an alcohol or water-based carrier solution, then sprayed, dipped, painted or rolled onto the surfaces of flat sheet or plate products to enhance their brightness.
GB 2 331 942 A relates to a composition comprising at least one organo phosphonate or organo phosphonate species used for the treatment of a variety of metal surfaces to inhibit corrosion and improve adherence of coatings. The composition can include a homopolymer or copolymer of an organo phosphonate or organo phosphonate species such as vinylphosphonic acid, vinylidene-1,1-diphosphonic acid or phenyl vinyl phosphonic acid.
U.S. Pat. No. 3,630,790 describes a method of protecting metal surfaces from corrosion which comprises contacting the metal with an organo phosphonic, phosphonous or phosphinic acid.
Although the above mentioned methods are all directed to treatment of metal and metal alloy surfaces in order to preserve their quality regarding solderability and appearance, there is still a need for an environmentally friendly process offering improved corrosion resistance as well as whisker suppression.